Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

The present invention provides a hologram recording material which is
suitable for volume hologram record and can attain high refractive index
change, flexibility, high sensitivity, low scattering, environment
resistance, that is, storage stability, durability, low dimensional
change (low shrinkage) and high multiplicity in holographic memory record
using not only a green laser but also a blue laser. A hologram recording
material comprising: a metal oxide matrix; and a photopolymerizable
compound which has at least one (meth)acrylamide group as a
photo-reactive group, and has a polyalkylene glycol unit represented by
the following formula:
--(RO)n--
wherein R represents a lower alkylene group, and n represents the number
of repeating units of alkylene oxide.

Claims:

1. A hologram recording material comprising:a metal oxide matrix; anda
photopolymerizable compound which has at least one (meth)acrylamide group
as a photo-reactive group, and has a polyalkylene glycol unit represented
by the following formula:--(RO)n--wherein R represents a lower
alkylene group, and n represents the number of repeating units of
alkylene oxide.

2. The hologram recording material according to claim 1, wherein the
photopolymerizable compound has a polyethylene glycol unit represented by
the following formula:--(CH2CH2O)n--wherein n represents
the number of repeating units of ethylene oxide.

3. The hologram recording material according to claim 1, wherein the
number of the repeating units is 3 or more.

4. The hologram recording material according to claim 1, wherein the metal
oxide matrix is formed from a matrix-forming material containing a metal
alkoxide compound and/or an oligomer thereof.

5. The hologram recording material according to claim 1, wherein the metal
oxide matrix is formed from a matrix-forming material containing a Si
alkoxide compound and/or an oligomer thereof.

6. The hologram recording material according to claim 1, wherein the metal
oxide matrix is formed from a matrix-forming material containing at least
one metal alkoxide compound selected from the group consisting of a Ti
alkoxide compound, a Zr alkoxide compound, a Ta alkoxide compound and a
Sn alkoxide compound, and/or an oligomer of the metal alkoxide.

7. The hologram recording material according to claim 1, wherein the metal
oxide matrix is formed from a matrix-forming material containing:a Si
alkoxide compound and/or an oligomer thereof; andat least one metal
alkoxide compound selected from the group consisting of a Ti alkoxide
compound, a Zr alkoxide compound, a Ta alkoxide compound and a Sn
alkoxide compound, and/or an oligomer of the metal alkoxide.

8. The hologram recording material according to claim 1, which further
comprises a photopolymerization initiator.

9. A hologram recording medium which has a hologram recording layer
comprising the hologram recording material according to claim 1.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to a hologram recording material
suitable for volume hologram recording, and a hologram recording medium
having a hologram recording layer made of the hologram recording
material.

[0003]2. Disclosure of the Related Art

[0004]Research and development of holographic memories have been advanced
as recording technique making it possible to attain large-capacity
recording and high-speed data transmission. Examples of the property
required for a hologram recording material include high refractive index
change at the time of recording, high sensitivity, low scattering,
environment resistance, that is, storage stability, durability, low
dimension change, high multiplicity, and the like.

[0005]As a hologram recording material, attention has been paid to an
organic-inorganic hybrid material, which is made mainly of an inorganic
matrix and a photopolymerizable monomer, and the hybrid material has been
investigated. The inorganic matrix is excellent in environment resistance
and durability.

[0006]For example, Japanese Patent No. 2953200 (Patent Document 1)
discloses an optical recording film wherein a photopolymerizable monomer
or oligomer and a photopolymerization initiator are contained in an
inorganic substance network film. As the photopolymerizable monomer,
disclosed is a monofunctional (meth)acrylic acid ester, a polyfunctional
(meth) acrylic acid ester, or the like (paragraph [0017]).

[0008]JP-A-2002-236439 (Patent Document 3) discloses a hologram recording
material comprising: a matrix made of an organic-inorganic hybrid polymer
obtained by copolymerizing an organometallic compound containing an
ethylenically unsaturated double bond and an organic monomer having an
ethylenically unsaturated double bond, as main chain constituting
components, and/or a hydrolyzed polycondensate thereof; a
photopolymerizable compound; and a photopolymerization initiator. As the
photopolymerizable compound, a photo-radical polymerizable compound or a
photo-cation polymerizable compound is described (paragraph [0041]); and
as the photo-radical polymerizable compound, a (meth)acrylic acid ester
monomer is disclosed (paragraphs [0042] to [0043]).

[0010]JP-A-2005-99612 (Patent Document 5) discloses a hologram recording
material comprising a compound having one or more polymerizable
functional groups, a photopolymerization initiator, and colloidal silica
particles. As one species of the polymerizable functional groups, a
(meth) acrylic acid ester monomer is disclosed (paragraphs [0019] to
[0022]). As a different example thereof, an acrylamide (for example,
ethylenebisacrylamide) is disclosed (paragraph [0023]).

[0011]JP-A-2005-321674 (Patent Document 6) discloses a hologram recording
material comprising: an organometallic compound at least containing at
least two kinds of metals (Si and Ti), oxygen, and an aromatic group, and
having an organometallic unit wherein two aromatic groups are directly
bonded to one metal (Si); and a photopolymerizable compound. As the
photopolymerizable compound, a radical polymerizable compound or a cation
polymerizable compound is described (paragraph [0039]); and as the
radical polymerizable compound, a (meth)acrylic acid ester is disclosed
(paragraph [0041]).

[0012]Patent Document 1: Japanese Patent No. 2953200

[0013]Patent Document 2: JP-A-11-344917

[0014]Patent Document 3: JP-A-2002-236439

[0015]Patent Document 4: JP-A-2005-77740

[0016]Patent Document 5: JP-A-2005-99612

[0017]Patent Document 6: JP-A-2005-321674

SUMMARY OF THE INVENTION

[0018]In the above Patent Documents 1 to 4, and 6, a (meth) acrylic acid
ester is disclosed as the photopolymerizable monomer. However, the
(meth)acrylic acid ester is easily hydrolyzed under an acidic or basic
condition. Moreover, the (meth)acrylic acid ester is easily subjected to
transesterification reaction by interaction between the ester and a metal
alkoxide compound or an oligomer thereof, which is a material for forming
a metal oxide matrix. Accordingly, when the (meth)acrylic acid ester is
used as a photopolymerizable monomer in a metal oxide matrix, the (meth)
acrylic acid ester is gradually decomposed so as to deteriorate the
hologram recording property of the recording material.

[0019]In the above Patent Document 5, the acrylamide (for example,
ethylenebisacrylamide) is exemplified as the photopolymerizable monomer.
However, ethylenebisacrylamide is low in hydrophilicity, and therefore,
is not evenly dispersed in a metal oxide matrix.

[0020]Any of the above-mentioned Patent Documents 1 to 6 disclose
holographic memory record using a green laser, but do not disclose
holographic memory record using a blue laser.

[0021]As the wavelength of a recording/reproducing laser is shorter, any
hologram recording layer is required to have a higher mechanical
strength, a higher flexibility, a higher homogeneity, and environment
resistance (that is, storage stability). If the mechanical strength of
the hologram recording layer is insufficient, an increase in the
shrinkage of the layer when recording is made or a fall in the storage
reliability is caused. In particular, in order to obtain a sufficient
contrast based on refractive index modulation by means of a
recording/reproducing laser having a wavelength in the short wavelength
region, it is preferred to make the microscopic mechanical strength high
up to some degree, and restrain monomer-shift and dark reaction after the
layer is exposed to light for recording. If the flexibility of the
hologram recording layer is insufficient, the shift of the
photopolymerizable monomer in the layer is hindered in recording so that
the sensitivity falls. If the homogeneity is insufficient, scattering is
caused at the time of recording/reproducing. Thus, the reliability of the
recording/reproducing itself deteriorates. An effect of the scattering
based on the insufficient homogeneity of the recording layer becomes
remarkable more easily in the case of a recording/reproducing laser
having a wavelength in the short wavelength region.

[0022]An object of the present invention is to provide a hologram
recording material which is suitable for volume hologram record and can
attain high refractive index change, flexibility, high sensitivity, low
scattering, environment resistance, that is, storage stability,
durability, low dimensional change (low shrinkage) and high multiplicity
in holographic memory record using not only a green laser but also a blue
laser.

[0023]The present invention includes the followings:

(1) A hologram recording material comprising:

[0024]a metal oxide matrix; and

[0025]a photopolymerizable compound which has at least one
(meth)acrylamide group as a photo-reactive group, and has a polyalkylene
glycol unit represented by the following formula:

(RO)n--

wherein R represents a lower alkylene group, and n represents the number
of repeating units of alkylene oxide.(2) The hologram recording material
according to the above-described (1), wherein the photopolymerizable
compound has a polyethylene glycol unit represented by the following
formula:

(CH2CH2O)n--

wherein n represents the number of repeating units of ethylene oxide.(3)
The hologram recording material according to the above-described (1) or
(2), wherein the number of the repeating units is 3 or more.(4) The
hologram recording material according to any one of the above-described
(1) to (3), wherein the metal oxide matrix is formed from a
matrix-forming material containing a metal alkoxide compound and/or an
oligomer thereof.(5) The hologram recording material according to any one
of the above-described (1) to (4), wherein the metal oxide matrix is
formed from a matrix-forming material containing a Si alkoxide compound
and/or an oligomer thereof.(6) The hologram recording material according
to any one of the above-described (1) to (5), wherein the metal oxide
matrix is formed from a matrix-forming material containing at least one
metal alkoxide compound selected from the group consisting of a Ti
alkoxide compound, a Zr alkoxide compound, a Ta alkoxide compound and a
Sn alkoxide compound, and/or an oligomer of the metal alkoxide.(7) The
hologram recording material according to any one of the above-described
(1) to (6), wherein the metal oxide matrix is formed from a
matrix-forming material containing:

[0026]a Si alkoxide compound and/or an oligomer thereof; and

[0027]at least one metal alkoxide compound selected from the group
consisting of a Ti alkoxide compound, a Zr alkoxide compound, a Ta
alkoxide compound and a Sn alkoxide compound, and/or an oligomer of the
metal alkoxide.

(8) The hologram recording material according to any one of the
above-described (1) to (7), which further comprises a photopolymerization
initiator.(9) A hologram recording medium which has a hologram recording
layer comprising the hologram recording material according to any one of
the above-described (1) to (8).(10) The hologram recording medium
according to the above-described (9), wherein the hologram recording
layer has a thickness of at least 100 μm.(11) The hologram recording
medium according to the above-described (9) or (10), wherein
record/reproduction of said hologram recording medium are performed using
a laser light having a wavelength of 350 to 450 nm.

[0028]The hologram recording material of the present invention contains a
specific photopolymerizable compound which has a (meth)acrylamide group
as a photo-reactive group, and has a polyalkylene glycol unit. Since the
specific photopolymerizable compound has a polyalkylene glycol unit which
is hydrophilic, the compound is good in compatibility with all of a metal
oxide matrix; and a metal alkoxide compound and an oligomer which is a
partially condensate of said metal alkoxide compound, at the stage of the
formation of the matrix. Thus, the photopolymerizable compound is
homogeneously dispersed in the metal oxide matrix. Since the specific
photopolymerizable compound has, as its photo-reactive group, a
(meth)acrylamide group which is stable against hydrolysis, the compound
is stable at all stages of a formation stage of a hologram recording
layer (a formation stage of the metal oxide matrix), at a storage stage
after the production of a hologram recording medium and before the medium
is exposed to light for recording, and another storage stage after the
medium is exposed to the light for the recording.

[0029]Accordingly, the hologram recording medium using the hologram
recording material of the present invention can give a sufficient
sensitivity and a sufficient refractive index modulation in
recording/reproducing using a blue laser as well as a green laser. The
medium is also excellent in storage stability after recording is made in
the medium. Furthermore, the hologram recording layer is so homogeneous
that light scattering is not caused when recording or reproducing is
made. As a result, the reliability of the medium for
recording/reproducing is high.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a view illustrating a schematic cross section of a
hologram recording medium produced in the example.

[0031]FIG. 2 is a plane view illustrating the outline of a hologram
recording optical system used in the example.

DETAILED DESCRIPTION OF THE INVENTION

[0032]The hologram recording material of the present invention is a
composition containing a metal oxide (namely organic-inorganic hybrid)
matrix and a photopolymerizable compound (namely monomer). The hologram
recording medium of the present invention has a hologram recording layer
comprising the hologram recording material. In the present specification,
a hologram recording layer may be called a hologram recording material
layer.

[0033]The metal oxide matrix can be formed by sol-gel reaction (that is,
hydrolysis and polycondensation) of a matrix-forming material containing
a metal alkoxide compound and/or an oligomer which is a partially
hydrolytic condensate of said metal alkoxide compound. The matrix is in a
gel or sol form. The metal oxide functions as a matrix or a dispersing
medium for the photopolymerizable compound in the hologram recording
material layer. In other words, the photopolymerizable compound in a
liquid phase is evenly dispersed with good compatibility in the gel- or
sol-form metal oxide matrix.

[0034]When light having coherency is irradiated onto the hologram
recording material layer, the photopolymerizable organic compound
(monomer) undergoes polymerization reaction in the exposed portion so as
to be polymerized, and further the photopolymerizable organic compound
diffuses and shifts from the unexposed portion into the exposed portion
so that the polymerization of the exposed portion further advances. As a
result, an area where the polymer produced from the photopolymerizable
organic compound is large in amount and an area where the polymer is
small in amount are formed in accordance with the intensity distribution
of the light. At this time, the metal oxide shifts from the area where
the polymer is large in amount to the area where the polymer is small in
amount, so that the area where the polymer is large in amount becomes an
area where the metal oxide is small in amount and the area where the
polymer is small in amount becomes an area where the metal oxide is large
in amount. In this way, the light exposure causes the formation of the
area where the polymer is large in amount and the area where the metal
oxide is large in amount. When a refractive index difference exists
between the polymer and the metal oxide, a refractive index change is
recorded in accordance with the light intensity distribution.

[0035]In order to obtain a better recording property in the hologram
recording material, it is necessary that a difference is large between
the refractive index of the polymer produced from the photopolymerizable
compound and that of the metal oxide. The refractive indices of the
polymer and the metal oxide may be designed so as to make any one of the
refractive indices high (or low).

[0036]When the metal oxide is prepared to have a lower refractive index
than that of the polymer produced from the photopolymerizable compound in
the design of the hologram recording material, it is advisable to use Si
at a relatively large content by percentage. On the other hand, when the
metal oxide is prepared to have a higher refractive index than that of
the polymer produced from the photopolymerizable compound, it is
advisable to use Ti, Zr, Ta, Sn or the like at a relatively large content
by percentage.

[0037]As described above, the metal oxide matrix may be formed from a
matrix-forming material which contains a Si alkoxide compound and/or an
oligomer (that is, partially hydrolytic condensate) thereof. The metal
oxide matrix may be formed from a matrix-forming material which contains,
besides the Si alkoxide compound and/or the oligomer thereof, at least
one metal alkoxide compound selected from the group consisting of a Ti
alkoxide compound, a Zr alkoxide compound, a Ta alkoxide compound and a
Sn alkoxide compound, and/or an oligomer (that is, partially hydrolytic
condensate) of the metal alkoxide compound.

[0038]The metal alkoxide compound as the matrix-forming material is
represented by the following general formula (I):

(R2)j M(OR1)k (I)

wherein R2 represents an alkyl group or an aryl group; R1
represents an alkyl group; M represents a metal; j represents 0, 1, 2 or
3, and k represents an integer of 1 or more provided that j+k is equal to
the valence of the metal M; and when R2s are present in accordance
with j, R2s may be different or the same, and when R1s are
present in accordance with k, R1s may be different or the same.

[0039]The alkyl group represented by R2 is usually a lower alkyl
group having about 1 to 4 carbon atoms. Examples thereof include methyl,
ethyl, propyl, isopropyl, butyl, isobutyl, and sec-butyl groups, and the
like. An example of the aryl group represented by R2 is a phenyl
group. The alkyl group and the aryl group may each have a substituent.

[0040]The alkyl group represented by R1 is usually a lower alkyl
group having about 1 to 4 carbon atoms. Examples thereof include methyl,
ethyl, propyl, isopropyl, butyl, isobutyl, and sec-butyl groups, and the
like. The alkyl group may have a substituent.

[0041]Examples of the metal atom represented by M include Si, Ti, Zr, Ta
and Sn. Other examples thereof include Ge, Al, Zn, and the like. In the
present invention, it is preferred to use at least two alkoxide compounds
represented by the general formula (I) containing Ms different from each
other, and it is preferred that one of two Ms is Si and the other metal
M, which is different from Si, is selected from the group consisting of
Ti, Zr, Ta and Sn. Examples of combination of the two metals include a
combination of Si and Ti, that of Si and Ta, and that of Si and Zr. Of
course, three metals may be combined with each other. The incorporation
of the two or more metals as constituent elements into the matrix makes
it easy to control characteristics of the metal oxide matrix, such as the
refractive index thereof; thus, the incorporation is preferred for the
design of the recording material.

[0042]Specific examples of the alkoxide compound (I) wherein M is Si
include tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane, in
each of which j=0 and k=4; methyltrimethoxysilane, ethyltrimethoxysilane,
propyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane,
propyltriethoxysilane, γ-mercaptopropyltrimethoxysilane,
γ-mercaptopropyltriethoxysilane,
γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane,
phenyltrimethoxysilane, phenyltriethoxysilane, and
phenyltripropoxysilane, in each of which j=1, and k=3;
dimethyldimethoxysilane, dimethyldiethoxysilane, and
diphenyldimethoxysilane, in each of which j=2, and k=2; and the like.

[0044]Furthermore, diphenyldimethoxysilane is preferred. When an
organometallic unit wherein two phenyl groups (Phs) are bonded directly
to one Si atom (Ph-Si-Ph) is incorporated into a metal oxide matrix, the
flexibility of the metal oxide matrix is improved and further the
compatibility thereof with the photopolymerizable compound, which will be
detailed later, or an organic polymer produced by the polymerization of
the compound becomes good. Thus, the incorporation of the organometallic
unit is preferred. Moreover, the refractive index of the metal oxide also
becomes high. The diphenylalkoxide compound of Si is easily available as
a starting material, and has good reactivity in hydrolysis and
polymerization. The phenyl groups may each have a substituent.

[0045]When a monoalkoxysilane (j=3 and k=1) such as trimethylmethoxysilane
is present, the polymerization reaction is stopped; thus, the
monoalkoxysilane can be used to adjust the molecular weight.

[0046]The alkoxide compound (I) of a metal M other than Si is not
particularly limited, and specific examples thereof include alkoxide
compounds of Ti, such as tetrapropoxytitanium, and tetrabutoxytitanium;
alkoxide compounds of Ta, such as pentaethoxytantalum [Ta(OEt)5],
and tetraethoxytantalum pentanedionate [Ta (OEt)4
(C5H7O2)]; and alkoxide compounds of Zr, such as
tetra-t-butoxyzirconium [Zr(O-tBu)4], and tetra-n-butoxyzirconium
[Zr(O-nBu)4]. Metal alkoxide compounds besides these examples may be
used.

[0047]An oligomer of the metal alkoxide compound (I) (corresponding to a
partially hydrolytic condensate of the metal alkoxide compound (I)) may
be used. For example, a titaniumbutoxide oligomer (corresponding to a
partially hydrolytic condensate of tetrabutoxytitanium) may be used. The
metal alkoxide compound (I) and the oligomer of the metal alkoxide
compound (I) may be used together.

[0048]The metal oxide matrix may contain an element other than the above
in a very small amount.

[0049]In the present invention, the photopolymerizable compound is a
photo-radical polymerizable monomer, and has, in the molecule thereof, at
least one (meth)acrylamide group as a photo-reactive group, and has a
polyalkylene glycol unit represented by the following formula:

(RO)n--

wherein R represents a lower alkylene group, for example, an alkylene
group having 1 to 4 carbon atoms, and n represents the number of
repeating units of alkylene oxide (that is, the weight-average
polymerization degree). The wording "(meth)acrylamide group" is a wording
for expressing a methacrylamide group and an acrylamide group
collectively.

[0050]Specific examples of the lower alkylene group include methylene,
ethylene, 1,3-propylene, 1,2-propylene, 1,4-butylene, and 1,3-butylene
groups. Of these groups, ethylene and 1,3-propylene groups are preferred
from the viewpoint of hydrophilicity. An ethylene group is more
preferred. In other words, the photopolymerizable compound is preferably
a compound having a polyethylene glycol unit:

(CH2CH2O)n--.

[0051]From the viewpoint of hydrophilicity and restraint of recording
shrinkage, the number n of the repeating units is preferably 3 or more,
more preferably 4 or more and 50 or less, even more preferably 5 or more
and 30 or less.

[0052]The photopolymerizable compound is classified into a monofunctional
compound, which has in the molecule thereof a single (meth)acrylamide
group, and a polyfunctional compound, which has in the molecule thereof
two or more (meth) acrylamide groups.

[0053]Since the specific photopolymerizable compound has a polyalkylene
glycol unit which is hydrophilic, the compound is good in compatibility
with all of a metal oxide matrix; and a metal alkoxide compound and an
oligomer which is a partially condensate of said metal alkoxide compound,
at the stage of the formation of the matrix. Thus, the photopolymerizable
compound is homogeneously dispersed in the metal oxide matrix. Since the
specific photopolymerizable compound has, as its photo-reactive group, a
(meth) acrylamide group which is stable against hydrolysis, the compound
is stable at all stages of a formation stage of a hologram recording
layer (a formation stage of the metal oxide matrix), at a storage stage
after the production of a hologram recording medium and before the medium
is exposed to light for recording, and another storage stage after the
medium is exposed to the light for the recording.

[0054]Accordingly, the hologram recording medium using the hologram
recording material containing the specific photopolymerizable compound
can give a sufficient sensitivity and a sufficient refractive index
modulation in recording/reproducing using a blue laser as well as a green
laser. The medium is also excellent in storage stability after recording
is made in the medium. Furthermore, the hologram recording layer is so
homogeneous that light scattering is not caused when recording or
reproducing is made. As a result, the reliability of the medium for
recording/reproducing is high.

[0055]One kind of the specific photopolymerizable compound may be used, or
two or more kinds thereof may be used together. In the present invention,
in the case of making the refractive index of the metal oxide high and
making the refractive index of the organic polymer low, the specific
photopolymerizable compound which forms the organic polymer is preferable
since the compound has no aromatic group to give a low refractive index
(for example, a refractive index of 1.5 or less).

[0056]The specific photopolymerizable compound which has in the molecule
thereof a single (meth)acrylamide group can be prepared, for example, by
dehydration from one equivalent of an N-hydroxyalkyl (meth) acrylamide
compound and one equivalent of a polyalkylene glycol compound, thereby
producing a mono(meth)acrylamide alkyl ether of polyalkylene glycol.

[0057]The alkyl group in the N-hydroxyalkyl(meth)acrylamide compound is
preferably, for example, an alkyl group having 1 to 20 carbon atoms. The
alkyl group may have a substituent. Specific examples of the
N-hydroxyalkyl(meth)acrylamide compound include
N-hydroxymethyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide, and the
like.

[0058]The polyalkylene glycol compound may be used in the form of a
monoalkyl ether thereof. In the case of using a monoalkyl ether of the
polyalkylene glycol compound, the alkyl group in the compound is
preferably, for example, an alkyl group having 1 to 20 carbon atoms. The
alkyl group may have a substituent. Specific examples of the polyalkylene
glycol compound include polyethylene glycol, polyethylene glycol
monomethyl ether, polyethylene glycol monoethyl ether, and the like.

[0059]Instead of the N-hydroxyalkyl (meth) acrylamide compound, an
N-hydroxyalkyl-N-alkyl(meth)acrylamide compound represented by the
following formula (II) may be used:

CH2═C(R5)--CON(R4)--R3OH (II).

In the formula (II), R5 represents a hydrogen atom or a methyl group,
--R3OH represents a hydroxy alkyl group, and R4 represents an
alkyl group. The alkyl group R4 is preferably, for example, an alkyl
group having 1 to 20 carbon atoms. The alkyl group R4 may have a
substituent, and R4 may have a hydroxyl group. When R4 has a
hydroxyl group, a compound having a single polyalkylene glycol chain is
obtained by using one equivalent of the polyalkylene glycol compound per
one equivalent of the (meth)acrylamide compound (II). When R4 has a
hydroxyl group, a compound having two polyalkylene glycol chains is
obtained by using two equivalents of the polyalkylene compound per one
equivalent of the (meth)acrylamide compound (II).

[0060]The specific photopolymerizable compound which has in the molecule
thereof two (meth)acrylamide groups can be prepared, for example, by
dehydration from two equivalents of an N-hydroxyalkyl (meth) acrylamide
compound and one equivalent of a polyalkylene glycol compound, thereby
producing a bis(meth)acrylamide alkyl ether of polyalkylene glycol.

[0061]As the N-hydroxyalkyl(meth)acrylamide compound, for example, the
same as in the case of the above-mentioned compound having in the
molecule thereof a single (meth)acrylamide group may be used. Instead of
the N-hydroxyalkyl(meth)acrylamide compound, the same
N-hydroxyalkyl-N-alkyl(meth)acrylamide compound (II) as described above
may be used.

[0062]The specific photopolymerizable compound which has in the molecule
thereof three or more (meth)acrylamide groups can be prepared, for
example, by synthesizing a polyethylene oxide adduct of a polyol compound
such as glycerol, erythritol or dipentaerythritol in advance, and then
subjecting this adduct and an N-hydroxyalkyl (meth) acrylamide compound
to dehydrating condensation, thereby producing the corresponding
(meth)acrylamide alkyl ether.

[0063]In the present invention, a (meth)acrylic acid ester monomer may be
used together with the above-mentioned specific photopolymerizable
compound as long as the attainment of the object of the present invention
is not damaged. The use amount of the (meth) acrylic acid ester monomer
should be set to, for example, an amount up to 10% by weight of the total
of the photopolymerizable compound(s).

[0066]It is advisable that in the present invention the photopolymerizable
compound is used, for example, in an amount of about 5 to 1,000% by
weight of total (as a nonvolatile component) of the metal oxide matrix,
preferably in an amount of 10 to 300% by weight thereof. If the amount of
the photopolymerizable compound is less than 5% by weight, a large
refractive index change is not easily obtained at the time of recording.
If the amount of the photopolymerizable compound is more than 1,000% by
weight, a large refractive index change is not easily obtained, either,
at the time of recording.

[0067]In the present invention, the hologram recording material further
contains a photopolymerization initiator corresponding to the wavelength
of recording light. When the photopolymerization initiator is contained
in the hologram recording material, the polymerization of the
photopolymerizable compound is promoted by the light exposure at the time
of recording. Consequently, a higher sensitivity is achieved.

[0068]Since a radical polymerizable compound is used as the
photopolymerizable compound, a photo radical initiator is used. Examples
of the photo radical initiator include Darocure 1173, Irgacure 784,
Irgacure 651, Irgacure 184 and Irgacure 907 (each manufactured by Ciba
Specialty Chemicals Inc.). The content of the photo radical initiator is,
for example, about 0.1 to 10% by weight, preferably about 0.5 to 5% by
weight on the basis of the photopolymerizable compound.

[0069]The hologram recording material composition may contain a dye that
functions as a photosensitizer corresponding to the wavelength of
recording light or the like besides the photopolymerization initiator.
Examples of the photosensitizer include thioxanthones such as
thioxanthen-9-one, and 2,4-diethyl-9H-thioxanthen-9-one; xanthenes;
cyanines; melocyanines; thiazines; acridines; anthraquinones; and
squaliriums. It is advisable to set an amount to be used of the
photosensitizer into the range of about 5 to about 50% by weight of the
radical photoinitiator, for example, about 10% by weight thereof.

[0070]A process for producing the hologram recording material will be
described in the following.

[0071]The metal oxide matrix may be prepared by causing a metal alkoxide
compound and/or an oligomer of the metal alkoxide to undergo hydrolysis
and polymerization reaction (the so-called sol-gel reaction).

[0072]The hydrolysis and polymerization reaction may be carried out by the
same operation under the same conditions as in known sol-gel methods. For
example, the reaction may be conducted by dissolving predetermined metal
alkoxide compound starting material(s) into a preferred organic solvent
to prepare a homogenous solution, adding an appropriate acid catalyst
dropwise to the solution, and stirring the solution in the presence of
water. The amount of the solvent is not limited, and is preferably 10 to
1,000 parts by weight with respect to 100 parts by weight of the whole of
the metal alkoxide compound.

[0074]The hydrolysis and polymerization reaction, which depends on the
reactivity of the metal alkoxide compound starting material(s), may be
conducted, in general, at room temperature (about 20 to 30° C.)
for 0.5 hour or more and 5 hours or less, preferably 0.5 hour or more and
3 hours or less. The reaction may be conducted in the atmosphere of an
inert gas such as a nitrogen gas, or may be conducted under a reduced
pressure of about 0.5 to 1 atm while an alcohol generated by the
polymerization reaction is removed.

[0075]Before, during or after the hydrolysis, the photopolymerizable
organic compound is mixed. The photopolymerizable organic compound may be
mixed with the metal alkoxide compounds as the starting materials after,
during or before the hydrolysis. In the case of the mixing after the
hydrolysis, it is preferred to add and mix the photopolymerizable organic
compound in the state that the sol-gel reaction system containing the
metal oxide and/or the metal oxide precursor is sol in order to perform
the mixing uniformly. The mixing of a photopolymerization initiator or
photosensitizer can also be conducted before, during or after the
hydrolysis.

[0076]The polycondensation reaction of the metal oxide precursor, with
which the photopolymerizable compound is mixed, is advanced to yield a
hologram recording material liquid wherein the photopolymerizable
compound is uniformly incorporated in a metal oxide matrix in a sol-form.
The hologram recording material liquid is applied onto a substrate, and
then drying of the solvent and a sol-gel reaction are further advanced,
thereby yielding a hologram recording material layer in a film form. In
such a way, the hologram recording material layer is produced wherein the
photopolymerizable compound is uniformly contained in a metal oxide
matrix.

[0077]The hologram recording medium of the present invention comprises at
least the above-mentioned hologram recording material layer. Usually, a
hologram recording medium comprises a supporting substrate (i.e., a
substrate) and a hologram recording material layer; however, a hologram
recording medium may be made only of a hologram recording material layer
without having any supporting substrate. For example, a medium composed
only of a hologram recording material layer may be obtained by forming
the hologram recording material layer onto the substrate by application,
and then peeling the hologram recording material layer off from the
substrate. In this case, the hologram recording material layer is, for
example, a layer having a thickness in the order of millimeters.

[0078]The hologram recording medium is either of a medium having a
structure for performing reproduction using transmitted light
(hereinafter referred to as a transmitted light reproducing type medium),
and a medium having a structure for performing reproduction using
reflected light (hereinafter referred to as a reflected light reproducing
type medium) in accordance with an optical system used for the medium.

[0079]The hologram recording medium of the present invention is suitable
for record and reproduction using not only a green laser light but also a
blue laser light having a wavelength of 350 to 450 nm. When the
reproduction is made using transmitted light, the medium preferably has a
light transmittance of 50% or more at a wavelength of 405 nm. When the
reproduction is made using reflected light, the medium preferably has a
light reflectance of 25% or more at a wavelength of 405 nm.

[0080]When the above described hologram recording material is used, a
hologram recording medium having a recording layer thickness of 100 μm
or more, which is suitable for data storage, can be obtained. The
hologram recording medium can be produced by forming the hologram
recording material in a film form onto a substrate, or sandwiching the
hologram recording material in a film form between substrates.

[0081]In a transmitted light reproducing type medium, it is preferred to
use, for the substrate(s), a material transparent to a
recording/reproducing wavelength, such as glass or resin. It is preferred
to form an anti-reflection film against the recording/reproducing
wavelength for preventing noises or give address signals and so on, onto
the substrate surface at the side opposite to the layer of the hologram
recording material. In order to prevent interface reflection, which
results in noises, it is preferred that the refractive index of the
hologram recording material and that of the substrate are substantially
equal to each other. It is allowable to form, between the hologram
recording material layer and the substrate, a refractive index adjusting
layer comprising a resin material or oil material having a refractive
index substantially equal to that of the recording material or the
substrate. In order to keep the thickness of the hologram recording
material layer between the substrates, a spacer suitable for the
thickness between the substrates may be arranged. End faces of the
recording material medium are preferably subjected to treatment for
sealing the recording material.

[0082]About the reflected light reproducing type medium, it is preferred
that the substrate positioned at the medium surface side into which a
reproducing laser light is irradiated is made of a material transparent
to a recording and reproducing wavelength, such as glass or resin. As the
substrate positioned at the medium surface side opposite to the medium
surface side into which a reproducing laser light is irradiated, a
substrate having thereon a reflective film is used. Specifically, a
reflective film made of, for example, Al, Ag, Au or an alloy made mainly
of these metals and the like is formed on a surface of a rigid substrate
(which is not required to have a light-transmitting property), such as
glass or resin, by vapor deposition, sputtering, ion plating, or any
other film-forming method, whereby a substrate having thereon the
reflective film is obtained. A hologram recording material layer is
provided so as to have a predetermined thickness on the surface of the
reflective film of this substrate, and further a light-transmitting
substrate is caused to adhere onto the surface of this recording material
layer. An adhesive layer, a flattening layer and the like may be provided
between the hologram recording material layer and the reflective film,
and/or between the hologram recording material layer and the
light-transmitting substrate. It is also unallowable that these optional
layers hinder the transmission of the laser light. Others than this
matter are the same as in the above-mentioned transmitted light
reproducing type medium.

[0083]The hologram recording medium which has the hologram recording layer
comprising the hologram recording material of the present invention can
be preferably used not only in a system wherein record and reproduction
are made using a green laser light but also in a system wherein record
and reproduction are made using a blue laser light having a wavelength of
350 to 450 nm.

EXAMPLES

[0084]The present invention will be specifically described by way of the
following examples; however, the invention is not limited to the
examples.

Example 1

Synthesis of an Acrylamide-Containing Monomer

[0085]From N-(hydroxymethyl)acrylamide and polyethylene glycol monomethyl
ether, a target compound (A) of monoacrylamide methyl ether/monomethyl
ether of polyethylene glycol represented by the following formula was
synthesized in accordance with a method described in JP-A-2006-225446:

CH2═CHCONHCH2OH+HO(C2H4O)nCH3→CH.-
sub.2═CHCONHCH2O(C2H4O)nCH3 (A).

[0086]A mixed solution of 100 parts by weight of
N-(hydroxymethyl)acrylamide (manufactured by Wako Pure Chemical
Industries, Ltd.), 266 parts by weight of polyethylene glycol monomethyl
ether (manufactured by Alfa Aesar Co. in UK, weight-average molecular
weight Mw=350), 0.3 part by weight of p-toluenesulfonic acid, and 0.2
part by weight of hydroquinone was heated to 60° C. in the flow of
nitrogen gas, and then was reacted for 10 hours. Next, the volatile
components were distilled off at 40° C. under reduced pressure.
Thereafter, an unreacted fraction of N-(hydroxymethyl)acrylamide was
removed by preparative liquid chromatography, so as to isolate a product.
The isolated product was dried under reduced pressure to obtain the
target compound (A). The structure of the compound was identified by
1H-NMR.

(Synthesis of a Matrix Material)

[0087]Mixed were 7.9 g of diphenyldimethoxysilane and 7.2 g of a titanium
butoxide oligomer represented by the formula illustrated below (B-10,
manufactured by Nippon Soda Co., Ltd.) to prepare a metal alkoxide mixed
liquid. The mole ratio of Ti/Si was 1/1.

[0088]A solution composed of 1.0 mL of water, 0.3 mL of a 1N aqueous
solution of hydrochloric acid, and 7 mL of 1-methoxy-2-propanol was
dropwise added to the metal alkoxide mixed liquid at a room temperature
while the mixed liquid was stirred. The resultant was continuously
stirred for 2 hours to conduct hydrolysis and condensation reaction. The
ratio of the metal alkoxide starting materials in the whole of the
reaction solution was 67% by mass. In this way, a sol solution was
obtained.

C4H9--[OTi(OC4H9)2]m--OC4H9,
wherein m=10.

(Photopolymerizable Compound)

[0089]To 100 parts by mass of monoacrylamide methyl ether/monomethyl ether
of polyethylene glycol (A) which was synthesized in the above-mentioned
step as a photopolymerizable compound, were added 3 parts by mass of a
photopolymerization initiator IRGACURE-907 (IRG-907, manufactured by Ciba
Specialty Chemicals K.K.), and 0.3 part by mass of thioxanthen-9-one as a
photosensitizer, so as to prepare a mixture containing the
photopolymerizable compound.

(Hologram Recording Material Solution)

[0090]The sol solution and the mixture containing the photopolymerizable
compound were mixed with each other at a room temperature to set the
ratio of the matrix material (as a nonvolatile component) and that of the
photopolymerizable compound to 67 parts by weight and 33 parts by weight,
respectively, to obtain a hologram recording material solution
substantially transparent and colorless.

(Hologram Recording Material)

[0091]With reference to FIG. 1, which schematically illustrates a cross
section of a hologram recording medium, explanation will be described.

[0092]A glass substrate (22) having a thickness of 1 mm and having one
surface on which an anti-reflection film (22a) was formed was prepared. A
spacer (24) having a predetermined thickness was put on a surface of the
glass substrate (22) on which the anti-reflection film (22a) was not
formed, and the hologram recording material solution obtained was applied
onto the surface of the glass substrate (22). The resultant was dried at
a room temperature for 1 hour, and then dried at 40° C. for 48
hours to volatilize the solvent. Through this drying step, the gelation
(condensation reaction) of the organic group-containing metal oxide was
advanced so as to yield a hologram recording material layer (21) having a
dry film thickness of 450 μm wherein the organic group-containing
metal oxide and the photopolymerizable compound were uniformly dispersed.

(Hologram Recording Medium)

[0093]The hologram recording material layer (21) formed on the glass
substrate (22) was covered with another glass substrate (23) having a
thickness of 1 mm and having one surface on which an anti-reflection film
(23a) was formed. At this time, the covering was carried out in such a
manner that a surface of the glass substrate (23) on which the
anti-reflection film (23a) was not formed would contact the surface of
the hologram recording material layer (21). In this way, a hologram
recording medium (11) was obtained which had a structure wherein the
hologram recording material layer (21) was sandwiched between the two
glass substrates (22) and (23).

(Evaluation of Characteristics)

[0094]About the resultant hologram recording material sample of Example 1,
characteristics thereof were evaluated in a hologram recording optical
system as illustrated in FIG. 2. The direction along which the paper
surface on which FIG. 2 is drawn stretches is defined as a horizontal
direction for convenience' sake.

[0095]In FIG. 2, the hologram recording medium sample (11) was set to make
the recording material layer perpendicular to the horizontal direction.

[0096]In the hologram recording optical system illustrated in FIG. 2, a
light source (101) for emitting a semiconductor laser (wavelength: 405
nm) in a single mode oscillation was used. Light emitted from this light
source (101) was subjected to a spatial filtrating treatment by means of
abeam rectifier (102), a light isolator (103), a shutter (104), a convex
lens (105), a pinhole (106), and a convex lens (107), so as to be
collimated, thereby enlarging the light into a beam diameter of about 10
mmφ. The enlarged beam was passed through a mirror (108) and a 1/2
wavelength plate (109) to take out 45° (45 degree) polarized
light. The light was split into an S wave and a P wave (the ratio of S
wave/P wave is 1/1) through a polarized beam splitter (110). The S wave
obtained by the splitting was passed through a mirror (115), a polarizing
filter (116), and an iris diaphragm (117) while a 1/2 wavelength plate
(111) was used to convert the P wave obtained by the splitting to an S
wave and then the S wave was passed through a mirror (112), a polarizing
filter (113) and an iris diaphragm (114). In this way, the total incident
angle θ of the two light fluxes irradiated into the hologram
recording medium sample (11) was set to 37°, so as to record
interference fringes of the two light fluxes in the sample (11).

[0097]The sample (11) was rotated in the horizontal direction to attain
multiplexing (angle multiplexing; sample angle: -21° to
+21°, angle interval: 0.6°), thereby attaining hologram
recording. The multiplicity was 71. At the time of recording, the sample
was exposed to the light while the iris diaphragms (114) and (117) were
each set to a diameter of 4 mm. At a position where the angle of the
surfaces of the sample (11) to the bisector (not illustrated) of the
angle θ made by the two light fluxes was 90°, the
above-mentioned sample angle was set to ±0°.

[0098]After the hologram recording, in order to react remaining unreacted
components, a sufficient quantity of blue light having a wavelength of
400 nm was irradiated to the whole of the surface of the sample (11) from
a blue LED. At this time, the light was irradiated through an acrylic
resin diffuser plate having a light transmittance of 80% so as to cause
the irradiated light not to have coherency (the light irradiation is
called post-cure).

[0099]At the time of reproduction, with shading by the shutter (121), the
iris diaphragm (117) was set into a diameter of 1 mm and only one light
flux was irradiated. The sample (11) was continuously rotated into the
horizontal direction from -23° to +23°. In the individual
angle positions, the diffraction efficiency was measured with a power
meter (120). When a change in the volume (a recording shrinkage) or a
change in the average refractive index of the recording material layer is
not generated before and after the recording, the diffraction peak angle
in the horizontal direction at the time of the recording is consistent
with that at the time of the reproduction. Actually, however, a recording
shrinkage or a change in the average refractive index is generated;
therefore, the diffraction peak angle in the horizontal direction at the
time of the reproduction is slightly different from the diffraction peak
angle in the horizontal direction at the time of the recording. For this
reason, at the time of the reproduction, the angle in the horizontal
direction was continuously changed and then the diffraction efficiency
was calculated from the peak intensity when a diffraction peak made its
appearance. In FIG. 2, reference number (119) represents a power meter
not used in this example.

[0100]At this time, a dynamic range M/# (the sum of the square roots of
the diffraction efficiencies) was a high value of 19.2, which was a
converted value corresponding to the case that the thickness of the
hologram recording material layer was converted to 1 mm.

[0101]The hologram recording medium sample of Example 1 was allowed to
stand still at a room temperature for one week, and then evaluated about
the recording/reproducing property thereof in the same way. As a result,
a dynamic range M/# was 18.8, which was substantially equal to the
initial value.

Comparative Example 1

[0102]A hologram recording medium was obtained in the same way as in
Example 1 except that a composition described below was used as a mixture
containing a photopolymerizable compound.

(Photopolymerizable Compound)

[0103]To 100 parts by mass of polyethylene glycol diacrylate (M-245,
manufactured by Toagosei Co., Ltd.) as a photopolymerizable compound were
added 3 parts by mass of a photopolymerization initiator IRGACURE-907
(IRG-907, manufactured by Ciba Specialty Chemicals K.K.), and 0.3 part by
mass of thioxanthen-9-one as a photosensitizer, so as to prepare a
mixture containing the photopolymerizable compound.

[0104]About the resultant hologram recording medium sample, the property
thereof was evaluated in the same way as in Example 1. The initial
dynamic range M/# was 17.9, which was a converted value corresponding to
the case that the thickness of the hologram recording material layer was
converted to 1 mm. However, after the sample was allowed to stand still
at a room temperature for one week, the dynamic range M/# was 10.5, which
was a largely deteriorated value.

Patent applications by Atsuko Kosuda, Tokyo JP

Patent applications by Jiro Yoshinari, Tokyo JP

Patent applications by Naoki Hayashida, Tokyo JP

Patent applications by TDK CORPORATION

Patent applications in class Composition or product or process of making the same

Patent applications in all subclasses Composition or product or process of making the same